Hitherto, as such a kind of pseudo-halftone processing method, e,crc/1/ an error diffusion method and e,crc/2/ an average density approximation method (Japanese Unexamined Patent Publication (Kokai) No. 57-104369) have been known.
The former is a binarizing method whereby multivalue pixel data of an object pixel is binarized using a predetermined threshold value and the errors between the binarized level and the multivalue pixel data of the object pixel are distributed to pixels near the object pixel by a predetermined distribution ratio and added. The latter is a method whereby the binarized data of pixels near the object pixel are used, the object pixel is binarized into black or white, weighted near-average values of both of the object pixel and the pixel near it are obtained, and an average value which is close to the object pixel level between the average values is selected, thereby binarizing the object pixel.
The former without involving a two-dimensional calculation for the multivalue data and has the drawback that the apparatus cannot be cheaply constructed by hardware due to the large amount of data processing required. According to the latter method, a texture of a low frequency which is peculiar to such a system is generated for an image having gentle density changes and the number of gradation levels which can substantially reproduced is extremely inferior to that of the former method.
Therefore, there is considered a novel binarizing method called an average density preservation method whereby a single average density is obtained and an image is binarized using the single average density value as a threshold value and a correcting process for writing binarization errors is added. With respect to the average density preservation method, the applicant of the present invention has already filed a new U.S. patent application, Ser. No. 07/476,766, on Feb. 8, 1990, on the basis of priority of Japanese Patent Applications Nos. 1-31404, 1-31405, 1-31408, and 1-31409 (all of were filed in Japan on Feb. 10, 1989) and a new U.S. patent application Ser. No. 07/476,618, filed on Feb. 7, 1990, on the basis of priority of Japanese Patent Application No. 1-31411 (filed in Japan on Feb. 10, 1989) and Japanese Patent Application No. 1-284879 (filed in Japan on Nov. 2, 1989).
However, when the density preservation type binarizing method is used, there is the drawback that halftone characters are made fine and become slightly blurred or dim, so that they cannot be clearly binarized.
On the other hand, there is also the drawback that notches of a size comparable to a pixel are generated in the edge portion of a character, a diagram, or the like, so that it is impossible to clearly binarize.